1. Field of the Invention
The present invention relates to a hierarchical random access method for a wireless communication system having a significantly large cell.
2. Description of Related Art
A random access method in a terrestrial mobile communication system may be based on a slotted ALOHA scheme. The random access method may be performed such that a time axis of the slotted ALOHA scheme is divided into slots, terminals are enabled to attempt a random access only where the slot starts, and when a receiver successfully receives a packet, an acknowledgement packet (ACK) is transmitted to solve a random access contention. Also, in the random access method, since the random access method does not transmit the ACK when a collision occurs, the terminals attempting the random access may wait for the ACK for a predetermined period of time, and then re-transmit the ACK when failing to receive the ACK.
The slotted ALOHA scheme may have an advantage in that a collision probability is relatively reduced in comparison with a general ALOHA scheme, and the random access method based on the slotted ALOHA scheme may be widely used in many mobile communication systems due to its simple implementation.
The random access method based on the slotted ALOHA scheme may be used in a Wideband Code Division Multiple Access (WCDMA) Universal Mobile Telecommunications System: (UMTS). Also, the random access method may enable access of terminals using 16 codes in a single arbitrary access slot.
In a case of a random access method based on a Single carrier-Frequency Division Multiple Access (SC-FDMA) scheme of a 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) scheme, a frequency (1.08 MHz) may be allocated for a random access channel unlike a Code Division Multiple Access (CDMA) scheme, a random access slot may be periodically (period: 10 ms) allocated in the allocated frequency, and 64-numbered Constant Amplitude Zero Auto-Correlation (CAZAC) sequences for each cell may be used in the allocated slot to perform random access.
In a method of designing a preamble for random access using the CAZAC sequence, four methods may be defined based on a cell size and environment. The four methods may be given using combination of a method of adjusting a length of a Cyclic Prefix (CP) with respect to the CAZAC sequence and a method of repeating an identical CAZAC sequence.
Also, in the random access method based on the SC-FDMA scheme of 3GPP LTE scheme, there is suggested a method in which a length of the CAZAC sequence used in the preamble for random access varies depending on a user location within a single cell to thereby transmit data. Based on the length of the CAZAC sequence allocated to the cell, in a case of a user existing near a base station, a relatively short part of the allocated length of the CAZAC sequence may be used without using all of the allocated length of the CAZAC sequence. In this method, the CAZAC may be repeated based on the short length of the CAZAC sequence to thereby configure a reference CAZAC sequence allocated to the cell.
A random access method in an existing terrestrial mobile communication system may design a length of a slot for random access and a preamble for random access based on a cell size, that is, a maximum distance between a base station and a mobile station.
However, in a case where the cell size is significantly large such as in a satellite communication system, since a time difference between signals received from terminals becomes larger, the length of the slot for random access may need to be lengthened. In this case, since a path loss becomes larger according to the cell size, existing systems (including the 3GPP LTE) may enable even a length of the preamble for random access (CAZAC sequence) to be lengthened to thereby obtain a signal gain.
In this manner, when the length of the preamble is lengthened together with the length of the slot, a case where the length of the slot needs to be increasingly lengthened due to the length of the preamble that may be created. This is because the length of the slot for random access is determined by adding the length of the preamble to a timing error of signals transmitted by the terminals.
Accordingly, the length of the slot for random access is significantly lengthened in a case of a significantly large sized cell, and thus a number of terminals being connectable per hour is reduced. Also, a Random Access Channel (RACH) method designed without considering various characteristics of the terminals may disadvantageously provide an environment in which performances of the terminals capable of providing better performances are not realized.